IABSE-JSCE Joint Conference on Advances in Bridge Engineering-IV, August 26-27, 2020, Dhaka, Bangladesh ISBN: 978-984-34-8313-3 Amin, Okui, Bhuiyan, Rahman (eds.) www.iabse-bd.org Soft ground shield tunneling in Bangladesh-application, construction challenges and moving forward M.N. Islam Multi-Lane Karnaphuli Tunnel Project (Bangabandhu Sheikh Mujibur Rahman Tunnel), Bangladesh Bridge Authority, Dhaka 1212, Bangladesh M. Rahman Bangladesh Bridge Authority, Dhaka 1212, Bangladesh M.H.R. Chowdhury Multi-Lane Karnaphuli Tunnel Project (Bangabandhu Sheikh Mujibur Rahman Tunnel), Bangladesh Bridge Authority, Dhaka 1212, Bangladesh S.V. Alcantara & S.J. Saju Multi-Lane Karnaphuli Tunnel Project (Bangabandhu Sheikh Mujibur Rahman Tunnel), SMEC-COWI JV ABSTRACT: Constructing tunnels using tunnelling shield is an almost 200 years old concept. However, tunnelling itself and its application in soft ground using slurry hydro-shield type tunnel boring machine (TBM), is a new construction methodology in Bangladesh. At 2450-meter length and 10.8meter in internal diameter twin tube road tunnel under the Karnaphuli River, is the first tunnel to be constructed in Bangladesh. There are several challenges for slurry pressure balance TBM tunnelling in soft soils – a tunnel built in soft ground such as clay, silt, sand or mud requires special technique compared to hard rock, to compensate for the shifting nature of the soil. This paper presents the relevant features, construction techniques, challenges in soft ground slurry shield tunnelling and solution for the construction challenges of the Karnaphuli Tunnel. 1 INTRODUCTION The Multi-Lane Road Tunnel under the River Karnaphuli or Karnaphuli Tunnel Project (KTP) is designed as a single-deck two-way four-lane tunnel with the East part and the West part of the tunnel constructed in two separate drives under the river or the tunnel is bored in two separate tubes. Utility corridors and escape routes are arranged under the upper deck, and three cross passages are arranged between the two tunnel tubes. In this report, construction difficulties and solutions of building the Karnaphuli Tunnel is discussed and a brief description of Shield Tunnelling Methodology is also included. It all starts at the working shaft, where complex geological condition was encountered and made the construction sluggish. Coupled with a prolonged rainy season, the underground water percolates with the river water and sea water. At the time of TBM launch, there was a risk that the saturated sand layer was prone to water and sand burst – resulting in seal failure. To mitigate these risks, prior to tunnelling, there was a need to strengthen and stabilize the ground where the tunnel boring machine (TBM) will break-in. So, a “sliding ring steel + freeze” method was adopted, where a tank-like steel sleeve will support the TBM breaking-in. Also, tri-axial mixing pile and Jet Grouting Pile (JGP) injection system was used in front of the working shaft to stabilize the ground forming a stabilized area. Finally, a ground freezing technique was utilized to reinforce the launching section. 2 OVERVIEW OF THE KARNAPHULI TUNNEL PROJECT (KTP) The Karnaphuli River divides Chittagong City into two –first piece is the area where main city and the seaport sits, the other piece is for the industrial areas. The part of the river that traverse the port, is heavily congested with ships and other vessels. The existing two bridges over the Karnaphuli River are already saturated with traffic movement. The Karnaphuli Tunnel Project will be used as a bypass route for traffic coming from Dhaka headed to Cox’s Bazar and the Karnaphuli Tunnel Project will reduce the traffic congestion of Chittagong Seaport as the trucks and lorry headed to go Cox’s Bazar from Chittagong sea port can bypass Chittagong City altogether. For Karnaphuli Tunnel Project (KTP),the tunnel is bored with 10.8-meter internal diameter and 11.8-meter external diameter, the ring width is 2000mm and ring thickness is 500mm. Precast reinforcement concrete common tapered segments are assembled with staggered joint. The lining rings consist of 8 segments where 5 258 standard segments (B), 2 adjacent segments (L), and 1 key segments (F). The ring segments uses three reinforcement type, designated as R1, R2 and R3 and adopting a C60 concrete strength with P10 impermeability. The tunnel is bored or mined using a pressurized slurry hydro-shield type Tunnel Boring Machine (TBM) over a length of 2450meter ground, starting on the West Bank near Patenga and ending in the East Bank near the Anowara. The two tunnels will be connected by three traversal cross passages at 700-meter interval with clear height of 2.6 meters for each cross passage. Refer to Figures 1 to 3. Two cut and cover and open cut sections are constructed at both ends of the tunnels. For retaining the earth in these structures, different solutions have been chosen. In the deepest part, diaphragm walls are used and in the shallow part steel sheet pile are used prior to excavation. To ensure safety from flooding on both banks during construction, temporary cofferdams was installed that can withstand a 20-year flood amount. Also, two flood gates would be installed at permanently at both ends of the tunnels. Total length of KTP including Tunnel section, cut and cover at both sides, open cut and approach roads is 9265 meters. The Karnaphuli River where it crossed by the tunnel is about 1240 meter wide. Commencement date of the KTP Project was 5th December 2017 and target completion on 4th December 2022. Figure 1. Site location of Karnaphuli tunnel project (Patenga, Chittagong, Bangladesh). Figure 2. Longitudinal section of left north tunnel. Figure 3. Cross section of Karnaphuli tunnel. 259 3 TUNNEL CONSTRUCTION 3.1 TBM Launching Priorto the launching of the tunnel boring machine (TBM), ground treatment using a triple pipe jet grouting method was adopted with 15meter distance from the tunnel portal. Also, at the starting position of shield, in order to reduce the risk of instability at the portal concrete wall and ensure the solidity of the tunnel face and to reduce the possibility of water leakage infiltration, freezing reinforcement wall within 1.5m was made at the shield starting end. The TBM Launching shaft was constructed using diaphragm wall and each launching shafts was a reinforced concrete thrust structure located at the back wall of the shaft to act as a reaction frame for advancing the TBM. Steel cradles and steel sleeve were installed to form a sealed chamber for retaining slurry during the start of the excavation. 3.2 Face Support Pressure During tunnel construction, soil is removed from the tunnel face. The soil layer in front and above the tunnel face exerts active earth pressure. The presence of infrastructures or surcharge also contributes as additional earth pressure. For the tunnel alignment below the groundwater table, water pressure is another significant component of pressure acting at the tunnel face. Establishing, and then maintaining the correct face support pressure (face pressure) for the ground and groundwater conditions is critical to the safe operation of slurry TBM. If inadequate face pressure is applied, this will lead to excessive ground movement, and may result in the collapse of the tunnel face. During the boring of the left line Karnaphuli Tunnel, the face pressure applied varies from approximately 0.5 bar to 5.5 bar. 3.3 Annual Grouting The annular space exists between lining segment and natural soil, and this space is filled up by primary grouting, thereby forming a peripheral waterproof layer. If it is determined to be insufficient, secondary grouting and subsequent compensation grouting should be done if necessary. The single-fluid grouting is used for primary grouting; cement paste is used for secondary or compensation grouting. Water glass or sodium silicate is used to treat temporary leakage and emergency treatment, to improve the durability of grout. To reduce shrinkage of grout, all grouting materials is mixed with micro-expansion agent. The grouting materials should possess good workability and dispersibility for water resistance as well as the appropriate gel time and strength, and its proportion of the mixture is determined by trial mixes. The grouting pressure is generally higher than the tunnelling hydraulic and face pressure by 1-2 bars, however, it should be adjusted according to the geological stratum and hydraulic pressure of the ground. In principle, the pressure should: (1) not be more than hydraulic and face pressure of excavated surface, (2) not cause uplift on the ground by more than 10mm, or further settlement of more than 30mm, (3) not dislocate or deform the segment from local pressure, (4) not allow slurry to leak frequently or in large quantity from the annulus between the segment clearance or shield machine and the segment. Lastly, proper measures were adopted to control the quality of grouting and the timeliness of grouting. 3.3.1 Grouting pressure control There are six slurry injection points at the tail of the shield. The annular grouting pressure of the tail is different due to the position of the slurry injection point. The construction should also be adjusted according to the actual conditions to achieve the balance between grouting pressure and the surrounding pressure. The pressure is roughly chosen to be equal to the sum of stratum resistance (pressure) plus 0.1 to 0.2 MPa. In addition, compared with the injected pressure earlier on, the post-injection pressure is 0.05-0.1 MPa larger than the injected pressure earlier on, and is used as a pressure management benchmark. The so-called resistance strength is the intrinsic value of the stratum, which is the minimum value of the pressure that the grout can be injected into the shield tail gap.